1. Field
The present invention relates to a method and apparatus for controlling a constant welding current for resistance welding.
2. Prior Art
A conventional alternating current resistance welding machine employs a pair of thyristors as contactors. The thyristors are connected to a primary coil of a welding transformer. A welding current from a secondary coil of the welding transformer flows through two overlapping workpieces, thus heating and melting them by Joule's heat into a metallurgically joined weldment.
Welding current control is achieved by controlling a thyristor firing angle. The principle is shown in FIGS. 9 and 10. In FIG. 9, the thyristor firing angle .phi. is set equal to a power factor angle .theta.. This results in an essentially continuous sinusoidal or "full heat" welding current I. In FIG. 10, the thyristor firing angle .phi. is greater than or lags the power factor angle .theta. by a delay time .zeta.. This causes discontinuity in the primary voltage of the welding transformer, resulting in a reduced and discontinuous welding current, a so-called "heat control" current. The greater the firing angle .phi., the longer the voltage rest interval and the smaller the welding current. In this manner, the firing angle control achieves control of the welding current magnitude. Thus, the constant-current control of the welding current may be achieved by controlling the thyristor firing angle .phi. such that the welding current I of each cycle matches the set welding current I.sub.0.
The prior art constant-current control system employs a current detector which is connected in a primary or secondary circuit of the welding transformer to measure a welding current. A feed-back control computes the error between the welding current measured in a previous cycle with the set welding current and determines the thyristor firing angle from the error such that the actual welding current will match the set welding current. However, for the first cycle of a welding operation, the prior-art constant-current control system turns on the thyristor at a predetermined firing angle since the feed-back control does not operate for the first cycle.
As mentioned, the prior art constant-current control system lacks a technique for correctly determining the thyristor firing angle at the beginning of a welding operation. Typically, it choses an initial thyristor firing angle which causes a reduced welding current to flow at the beginning cycles of the welding operation, as shown in FIG. 11(A). This method requires a welding time T.sub.A' which is longer than the desired time T.sub.A by .DELTA.T because of a delay in the heating of the workpieces. Such a delay slows the melting of the welding zone, and fatally damages the weld quality in the application of the projection welding. A contrary method is to choose an initial thyristor firing angle such that a welding current I exceeding the desired current I.sub.0 flows at the beginning cycles of the welding, as shown in FIG. 11(B). This, however, tends to generate a splash, yielding an unsatisfactory weldment.